Hyperacusis is an unusual intolerance to the loudness of an ordinary environment. Patients suffering from hyperacusis have an abnormally strong reaction to sounds within the auditory system that is manifested by inordinate discomfort to sound that would not evoke a similar reaction in the average listener. The estimated prevalence of hyperacusis in the general population is between about 0.6% and 15%. About 85%-90% of hyperacusis patients have an associated tinnitus condition. (Anari et al. 1999; Nelting 2002). It has been reported that up to about 55% of patients with tinnitus also have hyperacusis. (Schecklmann et al. 2014).
In some cases, patients suffering from hyperacusis exhibit symptoms of phonophobia which is a specific type of phobia in which the individual has a persistent, abnormal and unwarranted fear of sound. The resulting hypervigilance accounts for the exaggerated behavior in a patient's awareness to the sound environment.
Patients having hyperacusis may also exhibit symptoms of misophonia in which the individual experiences a strong, unpleasant reaction to ordinary sounds. This selective sensitivity to specific sounds may be accompanied by emotional distress and behavioral responses such as avoidance. In some instances, patients may also have pain associated with the hyperacusis. Neither hyperacusis, misophonia nor phonophobia have any relation to hearing thresholds. In hyperacusis, the auditory system is not working poorly but rather, it is working too well and overcompensating to a change in the normal gain setting of the auditory system.
Hyperacusis is associated with several peripheral and central auditory diseases as well as many non-auditory diseases. Exemplary disorders exhibiting hyperacusis symptoms include otosclerosis; efferent dysfunction; TMJ dysfunction; Bell's palsy; Meniere's disease; perilymphatic fistula; acute acoustic trauma; Lyme disease; autism; traumatic head injury; migraines; depression; childhood learning disability; diminished serotonin function; central auditory pathway lesions; William's syndrome; intracranial hypotension; myasthenia gravis; and Ramsey Hunt syndrome.
The auditory system is comprised of the outer ear, the middle ear and the inner ear as shown in FIG. 1. The outer ear is comprised of the pinna, the structure visible from the outside. It captures and directs sound waves into the ear canal, which in turn cause mechanical vibration of the eardrum. Behind the eardrum is the middle ear, which is an air-filled space that has a chain of 3 interconnected small bones. A sound wave vibrates the eardrum which vibrates the three bones. The third bone moves like a piston in and out of a membranous window at the entrance to the inner ear. The outer and middle ear facilitate sound transmission to the inner ear. The inner ear has 2 primary components: the balance system and the cochlea. The balance system has 3 semicircular canals that have nerve endings which send spatial information to the brain to balance and associate the body's position in space. The cochlea is the sensory organ for hearing. The cochlea is a hollow tube filled with fluid that spirals around and a membrane that has hair cells positioned on and spiraling around the tube. A hair cell has 3 parts: a main body (largest part), cilia at the top that are the tiny little hair fibers atop the main body and a connection to the nerve. Cilia can be thought of as mini hair cells that move back and forth when there's fluid motion inside the cochlea. When we hear a sound, there is movement of the fluid, bending of the membrane supporting the main hair cells, and bending of the cilia back and forth. This movement activates the big hair cells to send the auditory electrical signal via the nerve fibers to the brain. The hair cells transform mechanical energy (fluid motion) into an electrochemical signal so that the brain can process it.
There are two types of HCs: inner hair cells (IHCs) and outer hair cells (OHCs). IHCs are the primary sensory transducers of sound with 95% of the connecting neural fibers carrying auditory signals from the cochlea to the brain, where the signal is recognized as sound. Neural activity from IHCs go in one direction and receive little or no input back from the brain. The IHCs are sheltered and protected from damage more than the OHCs, which have a different function. The OHCs boost the strength of the auditory signal and fine tune it. The OHCs have the ability to amplify sounds instantaneously, up to 60 dB, to help the IHCs boost sounds if someone is talking softly or to attenuate sounds if they are loud. If the patient has hearing loss affecting the OHCs then this amplifying and fine tuning mechanism is diminished. OHCs send signals or impulses along the nerve fibers going up to the brain, but, unlike the IHCs, they have large numbers of nerve fibers coming from the brain back to the OHCs thus OHCs have control from the brain. This unique innervation characteristic suggests that the OHCs contribute to the instantaneous gain adjustment to sound, but the primary gain mechanism giving rise to hyperacusis is likely at the higher central level of the auditory system, above the cochlea. OHCS are very vulnerable to damage from noise, ototoxic drugs, viral infections and aging. Fortunately, OHCs are abundant; it has been estimated that humans could lose up to 30% of OHCs, spread evenly throughout the cochlea and still have normal puretone hearing thresholds.
The basilar membrane is frequency specific, meaning that different frequencies stimulate the cochlea at different locations. With regard to cochlear structure, the high frequency (HF) hair cells are located at the entrance of the cochlea where neural fibers are stimulated, and low frequency (LF) structures are stimulated and located distally to the entrance. The hair cells that respond to the higher frequencies, at the entrance to the cochlea, are more vulnerable to wear and tear which is why high frequency hearing loss is experienced more than low frequency hearing loss.
The nerve fibers in the auditory pathways cross over to stimulate both sides of the brain. This neural crossover from both ears facilitates localization of sound. This brings us up to the cortex, the cognitive center of the brain, where auditory signals are interpreted as sound. FIG. 2 highlights the complex auditory neural pathways arising from the cochlea, carrying auditory signals to the auditory cortex. This is the ascending pathway. There is an equally complex descending pathway from the auditory cortex back to the cochlea that is not shown here. The complex interactions between these two pathways control the gain of the auditory system.
There are 5 subcortical, lower levels within the central auditory pathways leading to the cortical brain that are responsible for filtering and enhancing information from the periphery. Each person weighs and processes information differently at these lower subcortical levels which also contribute to the central auditory gain.
These lower level structures work in different ways to affect the perception of sounds. One way is through selective perception in which the brain determines what is normal and filters out extraneous information that does not need attention such as refrigerator noise, the feel of our clothes, etc. The brain also processes information through sensory contrast in which the signal is examined in relation to its background. For example, a candle in dark room shines more brightly than in a sunny room. A third way the brain processes information is by prioritizing information and tasks in which the perceived importance of the signal is prioritized over the strength of the signal. Once the brain determines that the sound is not a threat and there is no danger, then it can override the information provided by the sound signal which permits the brain to intervene to change the perception of sounds. In the hyperacusis patient, the patient's system is too sensitive to all sounds, so it is unable to prioritize the specific important sounds. Perception happens in the cortical area, the primary auditory cortex, after processing the information from the subcortical areas.
The primary central gain mechanism presumably reflects processing in the subcortical areas. An example of the representation of “gain” is a dial that is normally set to 0. When the dial is turned clockwise there is an increase in central auditory gain, giving rise to the perception of a louder signal which is manifested by decreased LDLs consistent with hyperacusis. However, when the dial is turning back toward 0, the LDLs increase as the auditory gain is reduced and the signal is perceived to be less loud, which in turn results in expansion of the dynamic range. There is a constant low level of activity happening within the auditory pathways at all times. When everything is silent within the listening environment, there is an ongoing low level of neuronal activity. In hyperacusis, there is a form of neural hyperactivity ongoing. Gain is being adjusted (increased or decreased) in the subcortical areas. More specifically, some research suggests that the inferior colliculus has an important role in gain adjustment because this is where auditory information is first integrated by the two ears. Primary hyperacusis is almost always, if not solely, a bilateral problem and therefore an auditory pathway problem. The problem arises from abnormally elevated gain within the auditory pathways, which is represented by the reduced LDLs and reduced tolerance to sounds. Thus, whatever the cause or trigger of hyperacusis has resulted in increased auditory gain. If a patient wears earplugs to prohibit sound from coming into the system, the brain realizes that there is reduced or zero input and in response increases the gain of the system. This additional increase in the gain further exacerbates hyperacusis, resulting in even lower LDLs. When the system increases the gain, sounds are perceived louder than they were prior to use of sound-attenuating ear protection which is the reason earplugs are not effective as a treatment for hyperacusis.
In a normal-gain system, sound is transmitted to the peripheral ear (cochlea and nerve). (FIG. 3). As stated above, five subcortical areas of the brain process sound from the periphery of the ear and contribute to the gain system. Auditory activity is detected and filtered at the subcortical levels. If a signal is determined to be a neutral stimulus, these areas of the brain filter out the signal and send other important sounds to the auditory cortex with a normal gain, i.e. using the example enumerated above, the dial remains at 0. The auditory cortex then processes the subcortical information and the signal is perceived as sound.
In a hyper-gain system, sound is transmitted to the peripheral ear (cochlea and nerve) as in the normal-gain system. (FIG. 4). In the normal-gain system, neuronal processes in the auditory subcortical areas are operating in a state of equilibrium in terms of their excitatory and inhibitory actions. Abnormal changes in either set of actions can give rise to increased central auditory gain and hyperacusis. Moreover, if the subcortical areas view the signal as different, dangerous or new, then the subcortical processing enhances the associated neuronal activity, i.e. the dial registers this increase as a change in the gain setting. If the mechanisms that control auditory system gain abnormally amplify a signal, then the result is the perception of an abnormally loud sound in the auditory cortex thus resulting in primary hyperacusis.
There is no universal treatment for or proven cure for hyperacusis. Traditional treatments for hyperacusis include counseling such as hyperacusis activities treatment, social support and cognitive behavioral therapy or sound therapies such as sound attenuation and medication. (Pienkowski et al. 2014). The main goal of sound therapy is to expand the upper end of the dynamic range. Use of low-level broadband sound may be used to enhance sound tolerance. Although the underlying mechanisms are not now known, virtually all treatments using sound-enriching therapy implicitly or explicitly assume a recalibration or desensitization process by which controlled sound exposure gradually resets abnormally high auditory-pathway gain through an adaptive and plastic homeostatic neuronal process. Hazell and Sheldrake experimented with sound therapy using noise generators and increased LDLs in patients 8 to 10 dB over 2 to 6 months with 53% of patients showing treatment effects within 2 months and 73% of patients showing treatment effects within 6 months. The protocol was adopted into tinnitus retraining therapy (TRT). (Hazell & Sheldrake 1992).
In contrast, it has been found that hearing aids do not induce LDL changes. In analyzing the University of Maryland Tinnitus & Hyperacusis Clinic (UMTHC) records, it was found that the LDL change averaged 2.7 dB for 25 aided tinnitus patients with SNHL while matched groups who used noise generators for sound therapy showed changes from 5.9 to 10.1 dB. It was shown that patients with TRT+aided environmental sound have smaller LDL change than patients using noise generators for their sound therapy. In addition, patients using noise generators exhibited subjective improvements in sound tolerance and showed positive treatment effects regardless of the presence or absence of hearing loss, tinnitus and/or hyperacusis. (Formby et al. 2007).
Formby et al. examined the effect of counseling on patients having bilateral hearing loss with and without use of noise generators in a randomized, placebo-controlled study. It was found that treatment that included both counseling as well as noise generators was more efficacious than either treatment alone. (Formby et al. 2015)
Sammeth et al. developed a sound-limiting infinite compression device that was used for management of debilitating hyperacusis. The loudness suppression devices were housed in in-ear casings and supplied low-level amplification followed by an extreme form of amplitude compression for moderate or high level inputs to reduce loudness discomfort without reducing audibility. (Sammeth et al. 2000).
A typical hyperacusis patient may try using earplugs or ear muffs to limit exposure to sound levels that they consider loud. While the intent is to reduce the level of loud sounds, the effect of using such devices is full dynamic range attenuation which potentially exacerbates hyperacusis thus reducing LDLs over time. FIG. 5 illustrates the relative level change when using earplugs versus using a noise generator. Current best practices treatment focuses on noise generators and counseling; however, patients may have difficulty with the transition from earplugs/earmuffs to a noise generator device and fear any device or method related to amplification.
Currently fittings ignore patient-specific dynamic range. The LDL of non-hyperacusic patients can vary more than 30 dB. Current therapies prescribe the same gain for loud sounds for two patients with the same threshold even if their LDLs differ by 40 to 50 dB. Most fittings either over fit by assuming an LDL that is too high or under fit by assuming an LDL that is too low thus leading to discomfort, frustration, limited use and/or rejection.
The current treatment for hyperacusis includes the use of noise generators and counseling to expand the dynamic range, however an intermediate step with sound protection is needed. This step is counterproductive because wearing sound protection can exacerbate hyperacusis and prevents effective delivery of sound therapy. The invention described below overcomes a major dilemma—the patient's desire to wear sound protection rather than wearing and using a sound-therapy device. Given the shortcomings of current therapies for hyperacusis, what is needed is a therapy that is able to recalibrate the abnormal gain associated with hyperacusis by using the natural plasticity of the auditory system that is specifically tailored to the needs of the specific patient.